In recent years, mouse and human iPS cells have been established one after another. Takahashi and Yamanaka (non-patent document 1) induced iPS cells by transferring the Oct3/4, Sox2, Klf4 and c-Myc genes into fibroblasts from a reporter mouse wherein the neomycin resistance gene is knocked-in into the Fbx15 locus, and forcing the cells to express the genes. Okita et al. (non-patent document 2) succeeded in establishing iPS cells (Nanog iPS cells) that show almost the same gene expression and epigenetic modification profiles as those of embryonic stem (ES) cells by creating a transgenic mouse having the green fluorescent protein (GFP) and puromycin-resistance genes integrated into the locus of Nanog, whose expression is more localized in pluripotent cells than the expression of Fbx15, forcing fibroblasts from the mouse to express the above-mentioned four genes, and selecting puromycin-resistant and GFP-positive cells. Thereafter, it was revealed that iPS cells could also be produced with three of the factors other than the c-Myc gene (non-patent document 3).
Furthermore, Takahashi et al. (non-patent document 4) succeeded in establishing iPS cells by transferring into human dermal fibroblasts the same four genes as those used in the mouse. On the other hand, Yu et al. (non-patent document 5) produced human iPS cells using Nanog and Lin28 in place of Klf4 and c-Myc. Hence, it has been demonstrated that iPS cells comparable to ES cells in terms of pluripotency can be produced in both humans and mice, by transferring defined factors into somatic cells.
However, the iPS cell establishment efficiency is still low and, especially, a problem of extremely low iPS cell establishment efficiency occurs when human iPS cell is produced by introducing 3 factors (Oct3/4, Sox2 and Klf4) excluding c-Myc, which is feared to cause tumorigenesis in tissues or individuals differentiated from iPS cells, into somatic cells.
Ras, which is a small GTPase, regulates growth and differentiation in many cells. Ras is generally present as an inactivated form bound with GDP. When stimulated by a growth factor and the like, it dissociates GDP, binds to GTP to turn into an activated form, and transmits signal to the downstream via a target factor. As Ras target factor, Raf, phosphatidylinositol 3-kinase (PI3 kinase), Ral Guanine nucleotide Exchanging Factor (RalGEF) and the like are known. A constitutively activating point mutation of Ras has been reported in various human cancer cells, and therefore, functional collapse of Ras protein caused by abnormality in the downstream signaling by these target factors is assumed to be one of the important steps of cell canceration.
Takahashi et al. (non-patent document 6) identified a gene specifically expressed in embryonic stem cells (ES cells) and having a homology with other Ras genes and named it ERas. Although ERas shows only about 40% homology with other Ras as a whole, it highly conserves 5 guanine nucleotide-binding domains (G1-G5) essential for the function of Ras, and also has C-terminal Caax motif (C: cysteine, a: aliphatic amino acid, x: any amino acid) necessary for membrane localization.
However, the relationship between Ras family and reprogramming of somatic cell has not been sufficiently elucidated.